Expose ollama-roles default_chat for Assistent senior model default.

Write preferred chat tag into Assistent/ollama-roles.json so UI and warmup align with manifest default: true.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_ollama_model_use.py

@@ -33,6 +33,7 @@ models:
     roles = ollama_roles_payload(entries)
     assert roles["chat"] == ["chat-model:7b"]
     assert roles["memory"] == ["nomic-embed-text"]
+    assert roles["default_chat"] == "chat-model:7b"
 
 
 def test_ensure_memory_appended():